Okay so based on 10% we should do
1945 * 1.10 = 2139.5
Load Index 118 @ 45 = 2050
Load Index 118 @ 50 = 2270
2270 - 2050 = 220 / 5 = 44
2050 + 44 (+1 PSI) = 2094
2094 + 44 (+2 PSI) = 2138
45 + 2 = 47
So limited PSI should be set to 47? Is that right?
That's 5 more than I got without the +10% load capacity and would put me at the high end of the "+10-15 PSI" generalization.
The 10-15psi rule for same size works well but the details can get complicated.
No, the de-rating would lower your final air pressure if you followed it strictly as you would be trying to match a lower number. Is 1945 the load capacity of the LE tire at door sticker pressure? In any event, it would be that number divided by 1.10, however, I was just pointing out where the 44psi number came from and that a couple of psi here and there aren't real important.
Actually, using the linked Discount tire chart produces a psi more like 41 using the same de-rated method for 265/70/17. Since this chart produces lower numbers, I'd say skip the de-rating if using it. There are three tire standards organizations and while their numbers are very close to each other, they're not identical. Also, looking at various load/inflation tables, the load capacities of passenger tires of the same load index but different sizes shows slightly different numbers from tire to tire that seem to get a bit less consistent as you move away from max capacity, making me wonder if the linked chart is averaged. Maybe it's based on a newer method. Any of that could explain the slightly different load capacity numbers on the Standard Load (P) chart linked here, but, in any event, using load index with LTs can get confusing since a change in load index can mean either a size change or a load range change within the same size. With Ps, you know a change in load index means a change in size. I find the linked chart harder to use than just a regular load inflation table.
Back to your tire. You listed load index 118, I guess you have a load E and I'm presuming it's in the stock size which is 245/60/20, right? Using the above chart and assuming 32psi door sticker pressure, it looks to me like a de-rated method puts you at about 37psi (seems odd, but that's what it looks like). Using familiar load tables, about 41psi. Something to consider, though. You may have noticed that the load capacity of the LE stock tire is significantly lower than the 265/70/17. Might seem odd to have that much difference, but it's a higher speed rated tire with a nylon cap ply that offsets much of that difference so you're dealing with a little more than just adjusting for listed load capacity.
In other words, I'd still just use the 10-15 rule if the new LTs are same size.
